Although glass is an important material exhibiting transmittance with respect to light among materials constituting a building, since the glass has an extremely thin thickness and high density to secure transmittance as compared with a wall, heat insulation properties of glass are 1/10 or less those of a wall.
Since a typical sheet of glass has a thermal transmittance of greater than 5 W/m2K, there are a lot of difficulties in energy saving due to heat leakage during heating and cooling.
Recently, multilayer glass (pair-glass) prepared by supplementing heat insulation properties of single glass has attracted attention. Currently, general multilayer glass composed of two sheets of glass has a thermal transmittance of about 2.7 W/m2K when using glass to which a heat insulation coating is not applied, and can secure heat insulation properties corresponding to a thermal transmittance of up to about 1.3 W/m2K when using glass to which a low-emissivity coating is applied and an inert gas such as argon (Ar) and the like as a filling gas.
However, the multilayer glass still has high thermal transmittance as compared with a wall generally having a thermal transmittance from about 0.4 W/m2K to about 0.5 W/m2K. Recently, in the case of energy-saving houses, heat insulation properties corresponding to a thermal transmittance of glass of less than 0.7 W/m2K and to a thermal transmittance of 1.0 W/m2K in terms of a window including a window frame are required.
To satisfy such technical needs, vacuum glass capable of realizing heat insulation properties corresponding to a thermal transmittance of less than 0.7 w/m2K has been developed. However, since the vacuum glass is in a state in which a load of 7000 kg/m2 is applied to a glass surface due to maintenance of a vacuum of about 10−3 torr between two sheets of glass, the vacuum glass is extremely sensitive to external stress, such as external impact, temperature non-uniformity due to heat accumulation and the like, and thus has a great possibility of breakage.
In addition, recently commercially available triple-layer glass has a thermal transmittance of 1.0 W/m2K or more, which falls short of target heat insulation properties, and has a low heat gain coefficient and has a difficulty in securing comfortable sight since the triple-layer glass exhibits reduced light transmittance and increased reflectance due to the three sheets of glass included therein.
In the related art, Japanese Patent Laid-Open Publication No. H10-120447 (publication date: May 12, 1998) discloses multilayer glass, in which several sheets of pane glass use a spacer around overall edges thereof and are disposed at intervals in a thickness direction, and in which a low-emissivity coating is formed on an outer surface of at least one sheet of pane glass out of sheets of pane glass mounted on the outermost sides.